<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
		<style type="text/css">
		.box{
			list-style: none;
		}
		.box li{
			float: left;
			width: 70px;
			height: 30px;
			text-align: center;
			line-height: 30px;
			margin-right: 20px;
			
		}
		.box li::after{
			content: '';
			display: block;
			height: 0;
			width: 0;
			border-bottom: 1px solid #FF0000;
			transition: all 1s ease;
			margin: 0 auto;
		}
		.box li:hover::after{
			width: 70px;
		}
		</style>
	</head>
	<body>
		<ul class="box">
			<li>我是第一</li>
			<li>我是第二</li>
			<li>我是第三</li>
			<li>我是第四</li>
		</ul>
	</body>
</html>
